The present invention pertains to processes for pulping cellulosic materials, e.g., wood, bagasse and straw pulping.
In the manufacture of pulp from cellulosic materials for making paper, cardboard and related products the cellulosic fibers must be liberated from the cellulosic materials using liquid chemicals. The suspended solids of cellulosic fibers subsequently must be liberated and collected from the liquors.
The most common pulping processes employ cooking or digesting a cellulosic material, e.g. wood chips, to separate the cellulosic fibers from lignin and other materials in the wood chips. Side reactions of the pulping process produce high molecular weight organic by-products which include compounds of lignin, tar, pitch, saponified fatty acids and resins and other chemicals released from the wood chips.
During digesting processes, which can utilize a material, such as sulfate, caustic, sulfite, or alcohols, which is introduced into the digester along with the source of cellulosic fiber, with the digester raised to an elevated temperature and under pressure, the cellulosic fibers are released from the host material. Along with the fibers the high molecular weight organic by-products are released and become soluble in the liquid or pulping liquor. The high molecular weight organic by-products become dissolved or colloidal solids.
At the present time two commercial chemical processes are used to produce the cellulosic fiber material, i.e., Kraft or Sulfate Cooking and Acid or Sulfite Cooking. Another process that is the subject of promising laboratory work is Alcohol based Cooking (ASAM).
During and after the fiber producing material is cooked or digested and the fibers are released, the fibers are separated from the rest of the material and some or all of the spent cooking liquor can be used for further cooking of cellulosic material through circulating and recirculating the liquor. Dissolved solids in the spent liquor interfere in the initial cooking step or subsequent cooking steps. After the cooking process, the fiber is washed to remove residual cooking liquor and side reaction compounds. Further delignification can be obtained using oxygen and caustic which is commercially referred to as oxygen delignification within the washing sequence.
During the cleaning or washing processes during the manufacture of unbleached or bleached fibers, spent filtrate containing high molecular weight organic by-products can interfere with the physical and chemical processes needed to efficiently clean the fibers for the paper product. For example, a typical process of washing the fibers uses a cascading arrangement for the washing filtrate. Relatively clean water is introduced at the final washing stage and is cascaded back through several washing devices downstream to the first washing device. The washing equipment extracts the high molecular weight molecules which are carried on the fibers. The contaminated pulp, after washing yields a dirty filtrate which is sent to a preceding step in the washing process. Eventually, the high weight molecular organic by-products in the dirty filtrate interferes with efficient washing of the pulp. Also, residual high molecular weight material not removed from the fiber material interferes with a subsequent bleaching process.
Thus removal of the high weight molecular organic by-products will increase the efficiency of the cooking, washing, and bleaching processes involved in the production of pulp.